Chapter 2
Data Communications Concepts
What We’ll Be Covering
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Data Communications Concepts:
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Data Communications Architecture
Data Digitization
Data Transmission Techniques
Data Communication Techniques
Error Control Techniques
Overall Data Communications Architecture
Data Digitization
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The process of transforming humanly
readable characters into machine
readable code is character encoding.
Characters are turned into a series of
ones and zeroes (bits).
8 bits = 1 byte aka 1 octet
The most commonly used standards are
ASCII, EBCDIC, and UNICODE
7-Bit ASCII Code table
• ASCII encoding is used
on most computers
today
• ASCII table available
from Resources page
under the “Documents”
link
EBCDIC Code Table
• EBCDIC is used on IBM Mainframes
Unicode Table
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ISO 10646
standard
16 bits =
65,536
characters
Start  Accessories  System Tools  Character Map
Serial vs. Parallel Data Transmission
Serial vs. parallel Data Transmission
Transmission
Characteristic
Serial
Parallel
Transmission Description
One bit after another, one at a
time
All bits in a single character
transmitted simultaneously
Comparative Speed
Slower
Faster
Distance Limitation
Farther
Shorter
Application
Between two computers, from
computers to external devices,
local and wide area networks
Within a computer along the
computer’s busses, between a
drive controller and a hard drive
Cable Description
All bits travel down a single wire,
one bit at a time
Each bit travels down its own
wire simultaneously with other
bits.
Synchronous vs. Asynchronous
transmission
Half vs. Full duplex
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Data communications sessions are bidirectional in nature.
There are two environments available for
handling this bi-directional traffic: full and half
duplex.
In a full duplex communications environment
both devices can transmit at the same time.
In a half duplex environment you can only
hear or talk at any given point in time.
Given the choice of full or half duplex it is
usually better to choose full duplex.
Modulation vs. Demodulation
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This process is done by a mo(dulator)dem(odulator)
Modem Based Communication Channels
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The dial-up modem allows connections through the
phone network
Carrier Wave
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There are three properties of a wave that can
be modulated or altered:
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Amplitude
Frequency
Phase
Amplitude Modulation
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Each vertical lines separates opportunities to identify
a 1 or 0 from another.
These timed opportunities are known as signaling
events.
The proper name for one signaling event is a baud
Frequency Modulation
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frequency shift keying or FSK
Phase Modulation
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phase shift keying or PSK
Detecting Phase Shifting
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Quadrature Phase Shift Keying
Increasing Transmission Efficiency
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There are two ways in which a given
modem can transmit data faster:
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increase the signaling events per second,
or baud rate.
find a way for the modem to interpret more
than one bit per baud.
Differential Quadrature Phase Shift keying
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This technique improves transmission rate by
increasing the number of events per baud
Quadrature Amplitude Modulation
• Combines Amplitude
Modulation with Phase
Modulation
Data Compression
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Data compression techniques improve
throughput.
Data compression
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The sending device replaces strings of
repeating character patterns with a
special code that represents the pattern.
The code is significantly smaller than
the pattern it represents.
This results in the amount of data sent
between the sending device and the
receiving device to increase.
Packetization
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The process of dividing the data steam
flowing between devices into structured
blocks known as packets.
A packet is a group of bits organized in
a pre-determined, structured manner
consisting of a piece of the data stream
to which management information is
added.
Packetization
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This data stream is divided into 3 packets
Note the addition of header information to the data
portion
Packetization
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The predetermined structure of a
packet is critical.
Through the use of standards, devices
know the number of bits in each
section; the header, data portion and
trailer.
Determined by Network Architectures
(Ethernet) and Protocols (TCP/IP)
Encapsulation / De-encapsulation
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In a layered protocol, each layer adds a
header according to the layer’s syntax.
The sending device adds this
information in a process of
encapsulation
The receiving device reverses the this
process (de-encapsulation)
Encapsulation/De-encapsulation in the OSI model
Multiplexing
3 Types:
1.
Frequency Division
Multiplexing (FDM)
2.
Time Division
Multiplexing (TDM)
3.
Statistical Time
Division Multiplexing
(STDM)
Frequency Division Multiplexing
Time Division Multiplexing
Statistical Time Division Multiplexing
Switching
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Switching allows
temporary
connections to be
established,
maintained and
terminated
between sources
and destinations
Circuit Switching
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The work to create a signal path is done up
front; a switch fabric creates a direct path
between the source and the destination.
Communication takes place just as if the
temporary circuit were a permanent direct
connection:
The switched dedicated circuit makes it
appear to the user of the circuit as if a wire
has been run directly between the
communicating devices.
Packet switching
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In a packet switched network, packets of
data travel one at a time from the message
source to the message destination.
The physical path taken by one packet may
be different than that taken by other packets
in the data stream.
The path is unknown to the end user.
A series of packet switches pass packets
among themselves as they travel from source
to destination
Circuit vs Packet Switching
Datagram Delivery on a
Packet Switched Network
Connectionless vs. Connection-Oriented
Networks
Error Control Techniques
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Error Prevention
Error Detection
Error Correction
Flow Control
Error Prevention
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Reducing Noise & Interference on Lines
Improves Signal to Noise Ratio
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Filters
Amplifiers
Repeaters
Adaptive Protocols
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Transmission speed is adjusted based on
error rates
Error Detection Process
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The transmitting and receiving devices agree on how
the error check is to be calculated
The transmitting device calculates and transmits the
error check along with the transmitted data
The receiving device re-calculates the error check
based on the received data and compares its newly
calculated error check to the error check received
with the data
If the two error checks match, everything is fine. If
they do not match, an error has occurred
The Error Detection Process
Error Detection Techniques
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Parity (VRC)
Longitudinal Redundancy Checks (LRC)
Checksums
Cyclic Redundancy Checks (CRC)
Parity
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Parity, also known as a (Vertical
Redundancy Check or VRC), is the
simplest error detection technique.
Parity works by adding an error check
bit to each character.
Parity Checking
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Simple parity checking
Also known as Vertical Redundancy Check (VRC)
Parity Checking
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Parity checks can miss multiple bit errors
Longitudinal Redundancy Check (LRC)
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Longitudinal Redundancy Checks
(LRC) seek to overcome the weakness
of simple, bit-oriented one directional
parity checking..
LRC adds a second dimension to parity.
Longitudinal Redundancy Check (LRC)
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LRC improves parity checking at the cost of
extra data transmitted
Checksums
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Checksums are also block-oriented error detection
characters added to a block of data characters.
a checksum is calculated by adding the decimal face
values of all of the characters sent in a given data
block and sending only the least significant byte of
that sum.
The receiving modem generates its own checksum
and compares the locally calculated checksum with
the transmitted checksum
Checksum Calculation
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Add ASCII Decimal Value of Characters
Divide by 255
Remainder is the Checksum Character
Transmitted
If 128 Letter A’s are transmitted
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(A) 65 X 128 = 8,320
8,320/255 = 32 r160
160 in binary = 10100000
Cyclic Redundancy Check (CRC)
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Uses Binary Division
16-bit CRC uses a 17-bit divisor
32-bit CRC uses a 33-bit divisor
Error burst up to 1 bit less than CRC
can be detected 100% of the time
Larger error bursts at 100 - ½ CRC
percent of the time
Error Correction
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The receiving device detects the error
and requests a re-transmission
The sending device then retransmits the
portion that contained the error.
EC Protocol sophistication based on:
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How transmission is requested
How much data must be retransmitted
How retransmission time is minimized
Automatic Retransmission Request (ARQ)
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Discrete ARQ (ACK/NAK)
Continuous ARQ
Selective ARQ
Discrete ARQ (ACK/NAK)
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Also known as “stop and wait”
Receiver sends an Acknowledgement
(ACK) if no errors are detected
Receiver sends a Negative
Acknowledgement (NAK) if errors are
detected
If sender receives ACK it sends next block of
data
If sender receives NAK it retransmits original
block
Continuous ARQ
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Eliminates need for sender to wait for ACK or
NAK
Also known as “sliding window” protocol
Sequence number is appended to each block
of data sent
Receiver only sends NAK when error is
detected & returns sequence number
Sender slides back to sequence number and
begins retransmission
Selective ARQ
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Continuous ARQ requires retransmission
from point of error
Selective ARQ only requires
retransmission of sequence number
with error
Flow Control
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To prevent buffer overflows the receiving
device sends a signal to the sending device
The flow control software constantly monitors
the amount of free space available in buffer
memory and tells the sending device to stop
sending data when there is insufficient
storage space.
When the buffer once again has room the
sending device is told to resume transmitting